Telephone switching centers generally include switching networks which comprise a number of input and output terminals and a number of crosspoints. The individual crosspoints are selectively controlled to complete communication paths between the input and output terminals of the network. Historically, network crosspoints have comprised metallic contacts which were electrically, highly isolated from the control circuitry used to control them. With advances in semiconductor technology, semiconductor devices are being used in place of the metallic crosspoints. The use of such semiconductor devices tends to increase the speed of network operation and to reduce the size. Typical semiconductor crosspoints, however, allow a significant leakage current to flow from the control circuitry to a communication path that is established through a network of the crosspoints. This is true not only for the conductive crosspoints comprising the communication path but also for the many nonconductive crosspoints connected to that path. The leakage current that flows through each crosspoint varies with temperature, the voltage applied to the communication path and the particular device characteristics of that crosspoint. In addition, biasing resistors are typically distributed throughout the network to prevent the leakage current from so charging the communication paths that the crosspoints become conductive in an uncontrolled manner. The current which flows through these biasing resistors can vary even more significantly with the applied voltage than does the leakage current.
When voltage is applied via ideal test access lines to an open circuit, no current flows in those lines. The current which flows in a communication path of a semiconductor crosspoint network connected to an open circuited telephone subscriber loop due to the above-described leakage and biasing characteristics, also flows when any subscriber loop measurements are made through that network. Such current reduces measurement accuracy unless it is compensated for. Known compensation arrangements, which involves supplying fixed currents or connecting fixed negative resistances to communication paths, are unable to adequately account for the above-described variations with temperature and applied voltage or for the variations between different selected network paths when very accurate measurements are required. In view of the foregoing, a recognized problem in the art is the difficulty in making accurate measurements through a semiconductor crosspoint network having the above-described leakage and biasing characteristics.